Light emitting element and light emitting element array

ABSTRACT

A light emitting element includes a semiconductor including an active layer, and a planar shape of the light emitting elements including a concave polygon. The planar shape of the concave polygon has interior angles including at least one acute angle.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U. S. C. §119 to Japanese Patent Application No. 2013-236922, filed Nov. 15, 2013. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting element and a light emitting element array.

2. Discussion of the Background

Japanese Unexamined Patent Application Publication No. 10-326910 has proposed a triangular prism-shaped (FIG. 6 of this patent document) light emitting element and a truncated triangular pyramid-shaped (FIG. 10 of this patent document) light emitting element (see FIGS. 6 and 10 of this patent document). In this patent document, the following technique is described: that is, arranging outer surfaces of the light emitting element so as to form an acute angle therebetween reduces an angle, at which the light having undergone total internal reflection in the light emitting element is incident upon the next surface, and this can suppress repetition of total internal reflection in the light emitting element. The similar technique is described in claim 5 of Japanese Unexamined Patent Application Publication No. 2012-23249.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a light emitting element includes a semiconductor including an active layer and a planar shape of the light emitting element having a concave polygon. The planar shape of the concave polygon has interior angles including at least one acute angle.

According to another aspect of the present invention, a light emitting element array includes light emitting lements. Each of the light emitting elements include a semiconductor, and the semiconductor include an active layer, planar shapes of at least one of the light emitting elements include a concave polygon.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

FIG. 1 is a perspective view of a light emitting element according to an embodiment.

FIGS. 2A and 2B illustrate that, in the light emitting element according to the embodiment, the distance from where light is generated in the light emitting element to where the light is extracted to the outside of the light emitting element is reduced.

FIG. 3 explains that a tessellation can be formed by the single planar shape of the light emitting element according to the embodiment.

FIGS. 4A to 4C illustrate planar shapes of the light emitting elements other than that illustrated in FIG. 1 according to other embodiments.

FIGS. 5A to 5C illustrate comparison of a light emitting element array using the light emitting elements according to the embodiments and a related-art example of a light emitting element array.

FIGS. 6A and 6B illustrate examples of arrangements of the light emitting elements in the light emitting element arrays other than those illustrated FIGS. 5A to 5C, using the light emitting elements according to the embodiments.

FIG. 7 illustrates a method of cutting out the light emitting element in a method of manufacturing the light emitting element according to the embodiment.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention is described below with reference to the drawings. It is to be understood that the embodiment described below is an example for embodying the technical concept of the present disclosure and does not limit the present invention. Also, for clarity of the description, the sizes, positional relationships, and so forth may be exaggerated in the drawings. In principle, the same designations or the same reference numerals denote the same or similar elements, and duplicate description is omitted where appropriate.

Light Emitting Element

FIG. 1 illustrates a schematic perspective view of a light emitting element 100 according to a first embodiment of the present invention. Herein, the lower side (a first electrode 40 side, that is, a mounting surface side) of the present light emitting element as viewed in FIG. 1 is referred to as the “lower” side, and the upper side (a second electrode 30 side, that is, a side opposite to the mounting surface side) as viewed in FIG. 1 is referred to as the “upper” side.

As illustrated in FIG. 1, in the first embodiment, the planar shape of the light emitting element 100 is a concave polygon (polygonal shape having a reentrant angle). Out of the interior angles of the concave polygon, one or more interior angles are reentrant angles (angle larger than 180° and smaller than 360°) and one or more interior angles are acute angles. Herein, the planar shape refers to the shape or the outline of the light emitting element 100 when the light emitting element 100 is seen from above and below. With the one or more interior angles of the planar shape being acute angles, repetition of total internal reflection of light generated in the light emitting element 100 is suppressed in the light emitting element 100, and accordingly, light extraction efficiency can be improved. Furthermore, in comparison with the case where no reentrant angle is formed in the planar shape, when a reentrant angle is provided in the planar shape, the length of the propagation of the light to a position where the light is extracted to the outside can be reduced. Thus, absorption of the light by electrodes or other members in the light emitting element 100 can be suppressed (the details will be described later). That is, with the light emitting element 100, which has both the above-described features, the light extraction efficiency can be significantly improved.

As illustrated in FIG. 1, the light emitting element 100 includes a semiconductor part 20 on a substrate 10. The semiconductor part 20 includes a first semiconductor layer 21, an active layer 22, and a second semiconductor layer 23. A first electrode 40 can be provided on a lower surface of the substrate 10. The first electrode 40 can be light-reflective. A second electrode 30, which includes a transparent electrode 31 and a pad electrode 32, can be provided on an upper surface of the semiconductor part 20. The planar shape of the light emitting element 100 is a polygon having one or more acute interior angles and one or more reentrant angles.

Although the first electrode 40 is typically an n-electrode and the second electrode 30 is typically a p-electrode in the present embodiment, the first electrode 40 may be a p-electrode and the second electrode 30 may be an n-electrode.

FIG. 1 is drawn such that the external shape of the semiconductor part 20 matches those of the first electrode 40 and the transparent electrode 31. However, the external shapes of the first electrode 40 and the transparent electrode 31 may be actually smaller than that of the semiconductor part 20. Furthermore, regions near side surfaces where the active layer 22 is exposed may be coated with an insulating layer for preventing short circuit. Also, the tip of acute angle portions may be rounded for preventing chipping.

Referring to FIGS. 2A and 2B, in comparison with the related-art example, the effects of the light emitting element 100 is described. FIGS. 2A and 2B illustrate the length of the paths in plan view of the light generated in a point P in the light emitting element to the position where the light is extracted to the outside. FIG. 2A illustrates the light emitting element 100, which has a concave quadrangle planar shape. FIG. 2B illustrates the related-art example of the light emitting element having a convex triangular planar shape. When one of the interior angles is a reentrant angle as illustrated in FIG. 2A, the distances from the point P in the light emitting element to the sides of the polygon that form the reentrant angle, that is, the side surfaces of the light emitting element, can be reduced compared to the convex triangle as illustrated in FIG. 2B. By reducing the distances from a position where the light is generated to the side surfaces where the light is extracted, absorption of the light generated in the active layer 22 by the electrodes or the like in the light emitting element can be reduced. As a result, the light emitting element 100 with high light extraction efficiency can be realized.

Preferably, the planar shape of the light emitting element 100 is, among concave polygons, a concave quadrangle including three acute interior angles and one reentrant angle as illustrated in FIG. 1. When the planar shape of the light emitting element 100 is a concave quadrangle, all interior angles except for the reentrant angle can be acute angles. When the interior angles except for the reentrant angle are acute angles, incident angles, at which the light generated in the light emitting element 100 is incident upon the side surfaces, can be reduced in two surfaces forming an acute angle therebetween. Thus, the ratio of the light subjected to the total internal reflection in the light emitting element 100 to the light incident upon the side surfaces can be reduced, and the ratio of the light extracted to the outside to the light incident upon the side surfaces can be increased. That is, light extraction efficiency of the light emitting element 100 can be improved. Furthermore, since the concave quadrangle is a simpler shape compared to a concave pentagon or a concave hexagon, when the light emitting element 100 is cut out from the substrate 10 with the layers stacked on it (referred to as a wafer hereafter) in a manufacturing process, breakage or chipping can be suppressed compared to the case where the light emitting element has a concave polygon other than the concave quadrangle. Thus, the yield in the manufacturing process can be improved.

Preferably, the planar shape of the light emitting element 100 is a shape capable of forming a so-called tessellation, that is, the planar shape of the light emitting element 100 allows a planar configuration of multiple repetitions of the planar shape closely fit together, that is, substantially without leaving a gap or without overlapping. FIG. 3 illustrates an example in which a tessellation is formed by the single planar shape of the light emitting element 100. By forming the light emitting element 100 to have a planar shape capable of forming a tessellation, a wafer can be separated into a plurality of light emitting elements 100 reducing useless portions of the wafer when the light emitting element 100 is cut out from a wafer. Examples of a concave polygon capable of forming a tessellation other than the concave quadrangle include the following shapes: a hexagon having parallel opposite sides and having two reentrant angles (FIG. 4A, light emitting element 200); a shape obtained by dividing a hexagon having parallel opposite sides and two reentrant angles into two congruent concave polygons (FIG. 4B, light emitting element 300); and a shape formed by cutting part of a concave hexagon having parallel opposite sides and connecting a cut part of the concave hexagon to a remaining part of the concave hexagon such that a side of the cut part corresponds to another side of the remaining part which was parallel to the side of the cut part before being cut (FIG. 4C, light emitting element 400).

The materials and the film thicknesses of the components are not particularly limited. Known materials can be appropriately used. In the present embodiment, in order to form the semiconductor part 20 on the substrate 10, the substrate 10 is formed of a material suitable for epitaxial growth of the semiconductor part 20. Referring back to FIG. 1, the first electrode 40 is formed on a surface of the substrate 10 where the semiconductor part 20 is not formed, and the second electrode 30 is formed on the semiconductor part 20. In such a case, the substrate 10 interposed between both the electrodes is required to be electrically conductive. Thus, examples of the material of the substrate 10 include, for example, a nitride semiconductor (such as GaN), GaAs, GaP, Si, and SiC. For example, an about 300 μm thick n-type GaN substrate having the (0001) plane as a main surface can be used as the substrate 10.

More specifically, for example, a group III nitride compound semiconductor layer having a multilayer structure can be formed on the GaN substrate 10 as the semiconductor part 20. In the semiconductor part 20, for example, a 4.6 μm thick Si-doped n-type GaN layer can be used as the first semiconductor layer 21, a multiple quantum well structure (six wells) having well layers (3 nm thick, In_(0.3)Ga_(0.7)N) and barrier layers (25 nm thick GaN) can be used as the active layer 22, and a 20 nm thick Mg-doped p-type Al_(0.2)Ga_(0.8)N 0.2 μm thick Mg-doped p-type GaN layer can be used as the second semiconductor layer 23. As the first electrode 40, for example, Ti/Al/Ni/Au, Ti/Al, or Ti/Al/Ti/Pt/Al can be used. As the transparent electrode 31 of the second electrode 30, for example, 60 nm thick indium tin oxide (ITO) can be used. As the pad electrode 32 of the second electrode 30, for example, Pt/Au or Ti/Rh/Au can be used.

Various forms other than the form as illustrated in FIG. 1 are possible. Examples of the other forms can include, for example, the following forms: a flip-chip light emitting element that includes the substrate 10, which uses an insulating light-transmissive substrate formed of, for example, sapphire, and the first electrode 40 and the second electrode 30 formed on the surface of the semiconductor part 20 which is a different side from the side of the surface of the substrate 10; or a structure that include an electrically conductive adhesive layer, the second electrode 30, the semiconductor part 20, and the first electrode 40 on an electrically conductive non light-transmissive support substrate formed of, for example, CuW. Although the second electrode 30 includes the transparent electrode 31 so as to allow the light to be simultaneously extracted from the upper surface and the side surfaces of the light emitting element 100 in the present embodiment, for example, the following alternative structure may be used. That is, the first electrode 40 and the second electrode 30 are made to be optically reflective so as to allow the light to be extracted only from the side surfaces of the light emitting element 100.

Light Emitting Element Array

A light emitting element array can be obtained by arranging a plurality of the light emitting elements as described above and mounting the light emitting element array on a board. FIGS. 5A to 5C illustrate examples of the light emitting element array. FIG. 5A illustrates a light emitting element array 1000. In the light emitting element array 1000, an orientation of the reentrant angles of the light emitting elements 100 matches an arrangement direction of the light emitting elements 100. Herein, the “orientation of the reentrant angles” refers to an orientation along a line perpendicular to a line connecting the vertices of the salient angles adjacent to the reentrant angle (diagonal line that lies outside the concave polygon). The light emitting elements 100 are preferably arranged as follows: that is, as illustrated in FIG. 5A, a portion of another light emitting element is disposed in a region (region R) surrounded by the sides that form the reentrant angle of the concave polygon in the planar shape of the light emitting elements 100 and a line connecting the vertices of salient angles adjacent to the reentrant angle (diagonal line that lies outside the concave polygon). Here, the “salient angle adjacent to the reentrant angle” refers to an angle that one of the sides forming the reentrant angle makes with one of other sides of the concave polygon, at an end portion not having the reentrant angle. In other words, in the light emitting element array 1000, part of the other light emitting element is preferably disposed in a polygonal region, which is a region defined by subtracting a region inside the concave polygon from a region surrounded by lines connecting the vertices of all the salient angles of the planar shape of the light emitting element 100 other than the reentrant angle. Thus, when the light emitting element array is seen from the side surface side of the light emitting elements 100, it appears that there is no gap between the light emitting elements. This allows variation in brightness to be suppressed in a side surface direction when the light emitting element array emits light.

FIG. 5B illustrates a case of a light emitting element array 2000 formed by arranging the light emitting elements 400 as illustrated in FIG. 4C. In the planar shape of the light emitting element 400, two adjacent interior angles from among a plurality of interior angles are reentrant angles. Also in this case, part of another light emitting element is preferably disposed in the region R similar to that in FIG. 5A (part of the other light emitting element is disposed in a polygonal region, which is a region defined by subtracting a region inside the concave polygon from a region surrounded by lines connecting the vertices of all the salient angles other than the reentrant angles). Thus, the effects, which is similar to those produced by the light emitting element array 1000 illustrated in FIG. 5A, can be produced. The light emitting element array can be formed of the light emitting elements of a plurality of types of planar shapes. For example, a light emitting element array can be formed by arranging the light emitting elements 100 having the planar shape as illustrated in FIG. 1 and the light emitting elements 400 having the planar shape in which a plurality of reentrant angles are adjacent to each other as illustrated in FIG. 4C.

FIG. 5C is a related-art example of an array for comparison, in which related-art light emitting elements having a rectangular parallelepiped shape are arranged. In this case, light generated in the light emitting elements and extracted from side surfaces of the light emitting elements attenuates due to absorption by the adjacent light emitting elements while repeatedly reflecting between the side surfaces of the adjacent light emitting elements. In the cases of FIGS. 5A and 5B, the distance between each of the two sides that form the reentrant angle or reentrant angles of the light emitting element and a corresponding one of the adjacent two sides that form salient angles of the light emitting element increases as the distance from the reentrant angles increases. Thus, as indicated by arrows in FIGS. 5A and 5B, compared to the case where the distance between the light emitting elements are uniform (FIG. 5C), the number of times, by which the light extracted from the side surfaces of the light emitting elements is reflected between the side surfaces and the adjacent light emitting elements, can be reduced.

As an alternative form of the light emitting element array, as illustrated in FIG. 6A, the light emitting elements 100 can be arranged such that the orientation of the reentrant angles of the light emitting elements 100 is substantially perpendicular to the arrangement direction. In this case, by changing the reentrant angles, the amount of light that exits the reentrant angle side can be adjusted. Thus, distribution of light of the light emitting element can be asymmetric. This light emitting element can be preferably used for indirect lighting such as cove lighting. Furthermore, the light emitting elements 100 can be annularly arranged as illustrated in FIG. 6B. The light emitting elements in such arrangement can be preferably used as a light source of a spotlight.

Method of Manufacturing Light Emitting Element

An example of a method of manufacturing the light emitting element 100 according to the present embodiment will be specifically described below.

Semiconductor Part Growth Step

The semiconductor part 20 including the first semiconductor layer 21, the active layer 22, and the second semiconductor layer 23 is epitaxially grown on the substrate 10 (on the (0001) surface) by, for example, a metal-organic chemical vapor deposition (MOCVD) method. An orientation flat, which indicates the crystal orientation, is provided at an end of the substrate 10 such that a (10-10) surface of the GaN single crystal is exposed. In a crystal growth by the MOCVD method, hydrogen (H₂) or nitrogen (N₂) is used as a carrier gas. As group III materials, trimethyl gallium (TMG) or triethyl gallium (TEG) are used as a Ga source, trimethyl aluminum (TMA) is used as an Al source, and trimethyl indium (TMI) is used as an In source. As a group V material, ammonia (NH₃) is used as an N source. As the dopant, monosilane (SiH₄) is used as a Si material for the n-type, and bis-cyclopentadienyl magnesium (Cp₂Mg) is used as an Mg material for the p-type. By changing the supply amount of these material gases, the composition of each of the group III nitride compound semiconductor layers can be adjusted. The crystal growth can be performed under a pressure of, for example, around one atmosphere (around 100 kPa).

Step of Forming Second Electrode

The second electrode 30 is formed on the second semiconductor layer 23 by a known semiconductor wafer process techniques such as photolithography, reactive ion etching, chemical vapor deposition (CVD), sputtering, vapor deposition, lift-off, and annealing. Initially, the transparent electrode 31 is formed at a predetermined position on the second semiconductor layer 23 by typical photolithography, sputtering, and lift-off. Next, a groove having a width of about 50 μm is formed as an outer periphery of a substantially concave polygon by photolithography and reactive ion etching so that the first semiconductor layer 21 is exposed from the bottom of the groove. The concave polygon is, for example, the following concave quadrangle corresponding to that in FIG. 1: that is, as illustrated in FIG. 7, the interior angles are respectively 240°, 30°, 60°, and 30°, and the lengths of the sides are respectively about 1 mm, 0.58 mm, 0.58 mm, and 1 mm). By forming the light emitting element 100 to have the concave quadrangular planar shape and setting the long side of the concave quadrangle parallel to the orientation flat of the substrate, the side surfaces of the light emitting element 100 can be a {10-10} surface and a {11-20} surface. This allows the chip to be easily separated. Next, a resist having been used as a mask in the reactive etching is removed. After that, an insulating film of SiO₂ or the like is entirely formed on the resultant structure by plasma CVD method. Next, a contact hole is formed at part of the insulating film on the transparent electrode 31 by photolithography and reactive ion etching, and the pad electrode 32 is formed to have a specified shape so that the pad electrode 32 covers the contact hole.

Wafer Polishing Step

Next, the wafer, in which the second electrode 30 has been formed, is polished to adjust the thickness of the wafer. The wafer is bonded to a polishing holder, the lower surface of the wafer ((000-1) surface), which becomes a polished surface of the wafer, is ground by a grinder, and then the wafer is polished by a lapping machine with a polishing agent. The wafer polishing step is finished with a chemical mechanical polishing (CMP) performed on a polishing cloth with an alkaline aqueous solution. By performing the polishing step of multiple stages, the total thickness of the wafer becomes around 100 μm, and the lower surface of the wafer becomes a flat mirror surface.

Step of Forming First Electrode

The polished wafer is removed from the holder and then subjected to a process by a semiconductor wafer process apparatus. The first electrode 40 is formed at a predetermined position on the lower surface ((000-1) surface) of the substrate 10 of the polished wafer by typical photolithography, sputtering, and lift-off. Simultaneously, a region (scribe street) of about 50 μm wide, in which the first electrode 40 is not provided, is formed on the lower surface of the substrate 10 so as to oppose the groove at the outer periphery of the substantially concave quadrangle.

Singulation Step

Next, an adhesive sheet (dicing tape) is bonded to a metal ring (dicing frame, dicing ring, ring frame, ring) to hold (wafer mount) the wafer in which the first electrode 40 has been formed. The light emitting element 100 is cut out along the outline of the chip by laser processing. More specifically, the laser processing is performed by, for example, stealth dicing utilizing multiphoton absorption, laser ablation, deep grooving utilizing laser-induced backside wet etching (LIBWE) method, or a laser water jet. Then, the laser light is condensed by an objective lens optical system, part of the wafer along the scribe street is irradiated with the laser light focusing on the inside of the GaN substrate, multiphoton absorption is caused in the focus region, broken lines are drawn in four directions (for example, [−1010], [−2110], [−12-10], [01-10] or the like) by stealth dicing, in which a reformed region having a lower crystal strength than that before the irradiation is formed inside the GaN substrate.

Then, the laser light is focused on a deep position from the lower surface of the substrate and scanning is performed, and then, the laser light is focused on a shallower position from the lower surface and a second operation is performed so as to cut out a chip having a substantially concave quadrangular outline (see FIG. 7). At this time, when cleavage surfaces of the substrate are perpendicular to the active layer, the cleavage surfaces of the substrate 10 can be used as the side surfaces of the light emitting element 100. When cleavage surfaces of the substrate diagonally intersect the active layer 22, wafer singulation is performed in such directions that the cleavage surfaces are unlikely to be exposed. After that, the light emitting element 100 is separated by expanding.

With the method as described above, the light emitting element 100 of high lighting efficiency can be manufactured.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

What is claimed is:
 1. A light emitting element comprising: a semiconductor including an active layer, a planar shape of the light emitting element having a concave polygon, wherein the concave polygon has interior angles including at least one acute angle.
 2. The light emitting element according to claim 1, wherein the planar shape of the light emitting element allows a planar configuration of multiple repetitions of the planar shape closely fitted together.
 3. The light emitting element according to claim 1, wherein the planar shape of the light emitting element includes a concave quadrangle, and wherein interior angles of the concave quadrangle include one reentrant angle and three acute angles.
 4. The light emitting element according to claim 1, wherein the planar shape of the light emitting element includes a hexagon having parallel opposite sides, and wherein the hexagon has two reentrant angles.
 5. The light emitting element according to claim 1, wherein the planar shape of the light emitting element includes a shape obtained by dividing a hexagon having parallel opposite sides and two reentrant angles into two congruent concave polygons.
 6. The light emitting element according to claim 1, wherein the planar shape of the light emitting element is so constructed that a cut part of a concave hexagon having parallel opposite sides is connected to a remaining part of the hexagon such that a side of the hexagon in the cut part corresponds to another side of the hexagon of the remaining part which was parallel to the side of the cut part before being cut.
 7. A light emitting element array comprising: light emitting elements arranged to provide the light emitting element array, each of the light emitting elements including a semiconductor, the semiconductor including an active layer, planar shapes of at least one of the light emitting elements including a concave polygon, the concave polygon having interior angles including at least one acute angles
 8. The light emitting element array according to claim 7, wherein at least one of the light emitting elements including a region surrounded by sides forming a reentrant angle of the concave polygon and a line connecting vertices of salient angles adjacent to the reentrant angle; and a part of another of the light emitting elements being provided in the region.
 9. The light emitting element according to claim 7, wherein the planar shape of the light emitting element comprises a concave quadrangle, and wherein interior angles of the concave quadrangle include one reentrant angle and three acute angles.
 10. The light emitting element according to claim 8, wherein the planar shape of the light emitting element comprises a concave quadrangle, and wherein interior angles of the concave quadrangle include one reentrant angle and three acute angles.
 11. The light emitting element array according to claim 7, wherein the planar shape of the light emitting element is so constructed that a cut part of a concave hexagon having parallel opposite sides is connected to a remaining part of the hexagon such that a side of the hexagon of the cut part corresponds to another side of the hexagon in the remaining part which was parallel to the side of the cut part before being cut.
 12. The light emitting element array according to claim 7, wherein the light emitting elements are annularly arranged. 